Remove ecosystem_ci flag from Ruff CLI (#12596)

## Summary

@zanieb noticed while we were discussing #12595 that this flag is now
unnecessary, so remove it and the flags which reference it.

## Test Plan

Question for maintainers: is there a test to add *or* remove here? (I’ve
opened this as a draft PR with that in view!)

python/ruff-ecosystem/ruff_ecosystem/projects.py

@@ -221,7 +221,7 @@ class CheckOptions(CommandOptions):
         if self.exclude:
             args.extend(["--exclude", self.exclude])
         if self.show_fixes:
-            args.extend(["--show-fixes", "--ecosystem-ci"])
+            args.extend(["--show-fixes"])
         return args